Dielectric layers, such as oxides, are commonly used in semiconductor devices to electrically insulate one conductive member or conductive layer from another. For example in a transistor, a gate oxide is used to separate a gate electrode from a substrate material. In a non-volatile memory application, a tunneling oxide is used to separate a floating gate electrode from a substrate material. Oxides are also used frequently as capacitor dielectrics in semiconductor devices. In transistor, non-volatile memory, and capacitor applications, the oxide layer is very thin, typically on the order of 50-200 .ANG. (5-20 nm). Since the oxide layer is thin, any defects in the oxide will have a significant impact on the ability of the film to adequately insulate one conductor from another. Defects which might be present in an oxide may also alter a device's electrical characteristics, including breakdown voltage, programming voltage, and charge storage capacity. For this reason, it is important that thin oxides be of high quality and essentially free of defects.
Achieving a defect-free, thin oxide film is quite difficult. Defects may be introduced into an oxide film as a result of forming the film on a contaminated surface, forming the film in a contaminated reaction chamber, or simply as a result of the growth process in general. Furthermore, once an oxide film is formed, existing defects may be made worse and new defects may be created as a result of subsequent processing. Process induced damage to an oxide layer may be caused, for example, by ion implantation through the oxide layer or by exposing a device to a damaging plasma. Defects and process induced damage adversely affect the reliability of a thin oxide layer.
One known method of reducing the negative effects of defects in an oxide film is to form a CVD (chemical vapor deposition) film on top of the oxide, for instance a CVD silicon nitride (Si.sub.3 N.sub.4) film or a CVD oxide film. Depositing a CVD nitride film on a thermally grown oxide to form an oxide-nitride stack is a rather common approach to lowering defectivity. Furthermore, oxide-nitride stacks are more resistant to process induced damage in comparison to stand-alone oxides. Unfortunately, use of an oxide-nitride stack in transistors and capacitors has a significant disadvantage. Nitride films have a much higher charge trapping density than oxide films, resulting in a decrease in semiconductor device reliability, particularly as the nitride thickness increases.
A known alternative to a CVD oxide-nitride stack is a thermal oxy-nitride film formed by the nitridation of an oxide layer. Rather than being a composite of a nitride layer formed on an oxide layer, an oxy-nitride film is an oxide film which has undergone thermal nitridation, resulting in a film having a composition in the form of Si.sub.x O.sub.y N.sub.z. Like oxide-nitride stacks, oxy-nitride films have disadvantages which make the films less than desirable for use in semiconductor devices. In particular, processes used to form oxy-nitride films are not well suited for device fabrication. For instance, the thermal reaction required to form an oxy-nitride film occurs at a high temperature, usually at a temperature above 950.degree. C., although temperatures near 850.degree. C. have been reported. High temperature processing in semiconductor devices is undesirable, especially if diffused regions have previously been formed in an underlying substrate material since the high temperature will modify the diffusion profile in the substrate. Furthermore, the process tolerances for forming an oxy-nitride film are very small because parameters must be kept at levels which minimize the amount of nitrogen present at the interface between a substrate material and the initial oxide film. Too much nitrogen at this interface causes an undesirable shift in a device's threshold voltage and a reduction in transconductance.
In order to successfully scale down the size of semiconductor devices, an ongoing goal in the semiconductor industry, manufacturers must develop a method of producing thin, reliable oxide films. Each of the previously mentioned approaches to improving the reliability of a thin oxide has distinct and unfortunate disadvantages. For this reason a need exists for an improved method for forming dielectric layers in semiconductor devices.